1. Field of the Invention
The present invention relates to a two-wire audio conference apparatus, and more particularly to a two-wire audio conference apparatus having a howling and echo prevention function of which are generated in a case where a speaker and a microphone are positioned near each other.
2. Description of Related Art
FIG. 1 is a block diagram of a conventional two-wire audio conference apparatus. In FIG. 1, an echo estimator 004 estimates an echo signal when a part of an output signal to a speaker 001, i.e., a speaker output signal, goes around to a microphone 002 side. This estimated echo signal is subtracted from an input signal supplied from the microphone 002, i.e., a microphone input signal, by a subtracter 003 such that echo is canceled.
On the other hand, an echo estimator 011 estimates an echo signal when a part of a transmission output signal to a hybrid circuit 009 goes around to a reception input signal from the hybrid circuit 009. This estimated echo signal is subtracted from the reception input signal by a subtracter 010 such that echo is canceled.
A training controller 020 starts training in response to a trigger from an external trigger unit 019. Training is performed to each of the echo estimators 004 and 011. White noise generated from noise generators 015 and 014 are selected by selector 016 and 013 which are controlled by the training controller 020 and are outputted to the speaker 001 and the hybrid circuit 009 along the signal transmission directions, respectively. The echo estimators 011 and 004 refer to these signals to estimate in such a manner that generated echoes are canceled, respectively.
After sufficient estimation is completed, the echo estimator 011 is controlled to stop the estimating operation. The estimation operation of the echo estimator 004 is controlled by a bi-directional talk detector 017 to be described later. Also, the selectors 016 and 013 are reset to the original communication enable states.
A suppressing unit 018 compares the level of the reception input signal from the hybrid circuit 009 with the level of the microphone input signal. The suppressing unit 018 applies a predetermined fixed suppression between an input and an output on the side of lower level, but does not apply any suppression to the side of higher level. For example, it is assumed that level comparison between signals to input terminals a and c of the suppressing circuit 018 is performed and the level of the signal to the input terminal a is higher than that of the signal to the input terminal c. In this case, a suppression is applied to the signal to the input terminal c and the suppression applied signal is outputted from an output terminal b of the suppressing circuit 018. On the contrary, any suppression is not applied to the signal to the input terminal a and the signal to the input terminal a is outputted from an output terminal d as it is. This suppression allows howling to be prevented and generation of remaining echo after the echo cancellation to be prevented.
In the estimating operation of the echo estimator 004 which processes echo on the acoustic side, the level of the output signal to the speaker 001 and the level of the input signal from the microphone 002 are compared by the bidirectional talk detector 017. When the microphone input signal level is lower than the speaker output signal level, control is performed to activate the estimating operation of the echo estimator 004.
An amplifier 007 amplifies and adjusts the transmission output signal to a line in level. An amplifier 012 amplifies and adjusts a signal to the speaker 001 in level.
A first problem of this training method is that it is necessary to generate a large level of noise in training. When there is a far end apparatus having the same structure, the processes of noise generation from the speaker and noise sending to the line must be executed twice. Also, in this method, there is a possibility that the training is erroneously performed in the near end apparatus when the far end apparatus performs the training at the same time. For instance, when a signal is inputted from the far end apparatus to the near end apparatus while the near end apparatus is being trained, the estimation result of the echo estimator 011 would include an error.
The second problem of the conventional technique is that the training process needs to be performed at the beginning of communication for estimation by an echo estimator. Also, it is desirable that the sound source used for the training is white noise and has a high level. For this reason, the high level of noise in the training is unpleasant to the people nearby. Also, in a case of erroneous training, howling sometimes is generated.
The third problem of the conventional technique is that there is sometimes a remaining echo after the echo cancellation because of insufficient suppression so that there is a case where an echo returns. As a result, an echo is amplified from the speaker of the far end apparatus, which can degrade communication quality degrade. This is a problem especially when the line delay is large.